<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="./jquery-3.7.1.min.js"></script>
</head>
<body>
    <table border="1">
        <tr>
            <td>Row 1</td>

        </tr>
        <tr>
            <td>Row 2</td>
            
        </tr>
        <tr>
            <td>Row 3</td>
            
        </tr>
        <tr>
            <td>Row 4</td>
            
        </tr>
    </table>

    <script>
        $("tr:first").css("background-color","red")
    </script>
</body>
</html>